Professor John Campbell of the University of Birmingham, England, developed a series of rules for making reliable castings. One of the problems that he noted involves defects arising near the surface of non-ferrous metal castings. Because these defects are seen on surfaces of the casting that are in direct contact with the sand casting mold, these defects are commonly referred to as “metal mold reaction.” There are three distinct sub-divisions of the defects.
The first type is a gas porosity defect, that is, a porosity that is attributed to subsurface gas bubble formation. The gas can arise from several sources, including gas (notably hydrogen) dissolved in the molten metal, gas entrained in the molten metal during the pour, and gas from chemical breakdown or reaction of the components of the mold or core. This last category could include reaction of the reactive aluminum surface with atmospheric water on or near the molding materials.
The second type is a shrinkage porosity defect. Because the metal in contact with the mold surface will solidify first and more quickly, especially in metals with a small freezing range, shrinkage of the metal can occur, although this is likely to be away from the mold surface, but there may be influences seen there.
The third type of defect is the hot tear, which tends to take the form of a ragged, branching crack. Some alloys may have a higher propensity for tearing and some tearing may be truly random. As grains are formed during solidification, the separation of the grains can result in a tear.
Light metals such as aluminum and magnesium have important applications in automotive, marine and aerospace applications. These applications often involve the casting of thin pieces. However, a high integrity casting is required, as the presence of defects, especially porosity, will result in rejection of the casting.
The rate of cooling at the metal/mold interface has been recognized as an important factor in the quality of the cast part. Slower cooling generally results in decreased mechanical properties, indicated by increased microstructural length scales, such as dendrite arm spacing. Castings with a finer microstructure show better tensile and fatigue properties. Castings with smaller dendrite arm spacing values are consistent with smaller gas porosity and shrinkage porosity defects.
Some prior approaches to molding defects have concentrated on the binder composition that holds the molding sand together. For example, U.S. Pat. No. 6,288,139 to Skoglund (“Skoglund '139”) teaches a foundry binder system in which a Part I phenolic resin component and a Part II polyisocyanate component are used, where the Part II component contains from 0.1 to 5 weight % of an orthoester, the percentage being based upon the weight of the Part II component. Typically, these binder systems use the Parts I and II in a 55/45 weight ratio. Skoglund '139 recognizes that orthoesters have been known for stabilizing organic isocyanates, although the uses taught prior to Skoglund '139 did not extend to foundry binders and foundry mixes. When used in the Part II component, orthoesters were observed to improve tensile strength of the foundry shapes and the Part II components were observed to have lower turbidity at the time of use.
Another prior art approach is to add a composition directly to the alloy melt, with an intention of influencing or refining the grain structure in the casting. These “grain refiners” for aluminum include compounds such as titanium diboride (TiB2) (CAS 12045-63-5), potassium fluoroborate (KBF4) (CAS 14075-53-7) and potassium hexafluorotitanate (K2TiF6) (CAS 16919-27-0). Potassium fluoroborate is used as a sand additive in magnesium casting, but for a different reason. The compound inhibits the undesired formation of MgO, which can occur from reaction of hot magnesium with moisture or SiO2, by making fluoride anions available to form MgF2.
As useful as the known approaches have been, metal mold reaction, as particularly defined here, remains an ongoing problem for which the primary suggestion is to eliminate moisture from the mold when the metal pour occurs. Other factors may be influential. Improvement in the materials or techniques used is a desirable goal.